“I have no idea, but damn do those pants make your ass look huge.”
In dog years?
I usually say either “4” or “150”.
I don’t answer with an age because I’m terrible at that. I’m frequently off by decades.
If someone really presses me, I might try making comparisons. “You look older than her, but younger than her.” I’m reasonably good at figuring out older and younger when there are side-by-side comparisons, I just don’t know whether you’re 20 or 40.
First, I’d ask whether they really want an answer, and warn them that, while I’m not necessarily good at guessing ages, I’m going to do my best to give them an accurate guess.
If they say they want me to answer, I do exactly that - give them my best guess. If that upsets them, they learned a valuable lesson on the subject of ‘asking questions you don’t want the answers to’.
I always found 107 to be a pretty safe answer.
It’s a ridiculous question that I never ask…unless the other person started it by making a comment related to my apparent youth (I look ~30). It happens more often than you might think; frankly, I’m amazed by how frequently other people bring up my age. Sometimes I just let it go, but it’s usually more fun to respond with a smile and a “Really? How old do you think I am?”
Then I watch their heads explode when they find out I’m 45. 
When other people ask me, I generally decline to answer – something along the “oh, I’m horrible at guessing ages” lines.
“Old enough not to ask such a question.”
Twelve parsecs.
I always add ten years to whatever I think, unless it’s a teen, then I say twelve. I hate that game and won’t play it.
I absolutely hate being asked this. I’m no good at it, especially in Thailand, where many of the people look young well into middle age. I can never win, because even when I try to lowball it, I end up going so low they think I’m just being a joker. I’ve seen Thai ladies in their 40s who could pass for 25 or even younger in the West.
Once in Honolulu, I went with the future Mrs. Siam Sam and a few other Thai students to the popular dive bar near campus, Anna Bananas, for some live music. One was a 25-year-old doctoral student in economics. She’d forgotten her ID, and they wouldn’t let her in because they truly believed she was a high-school student. She had to go back and get it. Seeing her 25 years later, she has a high-level position at the Bank of Thailand, the country’s Federal Reserve, and she looks pretty much the same!
[QUOTE=ToughLife]
How do you respond ?
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Just smile and say that you’re terrible at estimating ages and would not want to try. End of story.
If the sign says “Land mines ahead! Danger! Peligroso!” do you just charge on ahead?
I work in aging, so I might say something like “way too young to need my help yet!” It doesn’t matter if they are 2 or 100.
ETA: The only people who ever ask me this are really old, by the way. I’ve never been asked by anyone else that I can remember, but I’d give the same answer.